Tackifier resins typically have a low number average molecular weight (M.sub.n) for compatibility and a high glass transition temperature (T.sub.g) for strength. Blending a tackifier resin with a higher molecular weight elastomer or base polymer, generally results in a blend that is particularly desirable in adhesive and sealant applications. The tack and adhesion properties of various base polymers can be significantly improved by combining the base polymer with a tackifier resin to produce a blend having a lower number average molecular weight and a higher glass transition temperature (T.sub.g) than the unblended base polymer. Tackifiers can be synthesized to contain only carbon and hydrogen, as in petroleum resins and polyterpenes, or can also contain oxygen such as coumarone-indenes or can be recovered from naturally occurring substances such as rosin esters.
Tackifiers are typically low molecular weight hydrocarbon resins produced by the polymerization of various feedstocks, typically olefin, diolefin, aliphatic, aromatic, or mixtures thereof. Typical polymerization systems include carbocationic and thermal polymerization.
Carbocationic polymerization of tackifier resins typically uses a Friedel-Crafts catalyst to polymerize aliphatic and/or aromatic monomers, such as disclosed in U.S. Pat. No. 3,966,690. Typical aliphatic monomers are C.sub.5 to C.sub.6 paraffins, olefins, and diolefins. Typical aromatic monomers are alkylated benzenes or higher aromatics containing at least one vinylic unsaturation. However, because of the low catalyst activity levels combined with high catalyst to monomer ratios attributable to a Friedel-Crafts process, this process requires additional processing steps to purify the resin. These additional process steps result in aluminous waste water and chlorinated organic by-products which must be accounted for in the overall production costs. Furthermore, Friedel-Crafts produced resins typically result in relatively high Gardner color values of greater than 2. A lower color is desirable for many commercial applications. Thus, in order to reduce the color of the resins, further costly processing such as hydrogenation is required prior to use.
Thermal polymerization of tackifiers is typically a Diels-Alder reaction of cyclopentadiene and/or dicyclopentadiene derivatives, and optionally C.sub.4 -C.sub.5 acyclic conjugated dienes and/or alkyl aromatic monomers containing at least one polymerizable group. Thermal polymerization takes place without a catalyst, therefore eliminating the costs associated with aluminous water and chlorinated by-products. However, tackifier resins produced by thermal polymerization have a high degree of unsaturation resulting in a Gardner color typically greater than 5. Therefore, to meet many commercial requirements, additional costly processing steps are required to reduce the color of the resins. For additional background on traditional hydrocarbon resins useful as tackifiers, see Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., 1995, vol. 13, pp. 717-743.
Until now useful tackifiers have not typically been produced using coordination catalysis. Coordination catalysis, such as traditional Ziegler-Natta polymerization systems, have not been used to produce polymers having both a low M.sub.n and a high T.sub.g generally needed in tackifier applications. Titanium-based catalyst systems using organoaluminum cocatalysts are known to copolymerize ethylene and cyclic-olefins. However, these titanium-based processes suffer from inefficiencies in comonomer incorporation and the presence of side reactions, such as ring opening polymerization of the cyclic-olefin, resulting in copolymers having a broad molecular weight distribution. Vanadium-based catalyst systems using organoaluminum cocatalysts offer improved comonomer incorporation relative to titanium, but because the polymerization activity is generally very low, these types of catalyst are not typically used commercially.
U.S. Pat. No. 5,059,487 discloses a hot melt adhesive comprising a vinyl-aromatic compound/conjugated diene copolymer, an alicyclic hydrocarbon resin tackifier, and a random cyclic copolymer of ethylene and a cyclic olefin. The cyclic copolymer is not a tackifier as it is added to improve high temperature performance and can only be added to the adhesive in limited quantities since it increases the viscosity of the adhesive.
U.S. Pat. No. 5,003,019 discloses the use of a metallocene catalyst system using an alumoxane activator to produce copolymers of cyclic-olefins and alpha-olefins, but demonstrate only high M.sub.n thermoplastics. EPA 0 504 418 A1 discloses a metallocene coordination catalyst system to produce homopolymers and copolymers of cyclic-olefins with either very high or very low T.sub.g 's, but demonstrate only homopolymers or high M.sub.n copolymers either of which have little value for typical tackifier applications.
Thus, the need exists for novel tackifiers having a desirably low M.sub.n and high T.sub.g while offering the further advantage of possessing good color. Furthermore, a need exists to produce low color tackifier resins without the need for additional costly processing steps. A production process minimizing aluminous waste water and chlorinated organic by-products would also be desirable.